The design and development of a solar tracking unit

The solar tracking unit was developed to support the Laser Heterodyne Spectrometer (LHS) airborne instrument, but has application to a general class of airborne solar occultation research instruments. The unit consists of a mirror mounted on two gimbals, one of which is hollow. The mirror reflects a 7.6 cm (3.0 in.) diameter beam of sunlight through the hollow gimbal into the research instrument optical axis. A portion of the reflected sunlight is directed into a tracking telescope which uses a four quadrant silicon detector to produce the servo error signals. The colinearity of the tracker output beam and the research instrument optical axis is maintained to better than + or - 1 arc-minute. The unit is microcomputer controlled and is capable of stand alone operation, including automatic Sun acquisition or operation under the control of the research instrument

Study of foldable elastic tubes for large space structure applications

Various modifications were made to the tube design. The tubes were retested and analyzed, and the results are presented. One type of modified tube, the slotted tube, deployed successfully and reliably, and became the focus of detailed tests. Optimal design criteria, taking into consideration deployment as well as strength and buckling behavior were established

Simulating Electron Transport and Synchrotron Emission in Radio Galaxies: Shock Acceleration and Synchrotron Aging in Three-Dimensional Flows

We present the first three-dimensional MHD radio galaxy simulations that explicitly model transport of relativistic electrons, including diffusive acceleration at shocks as well as radiative and adiabatic cooling in smooth flows. We discuss three simulations of light Mach 8 jets, designed to explore the effects of shock acceleration and radiative aging on the nonthermal particle populations that give rise to synchrotron and inverse-Compton radiations. We also conduct detailed synthetic radio observations of our simulated objects. We have gained several key insights from this approach: 1. The jet head in these multidimensional simulations is extremely complex. The classical jet termination shock is often absent, but motions of the jet terminus spin a ``shock-web complex'' within the backflowing jet material of the head. 2. Understanding the spectral distribution of energetic electrons in these simulations relies partly upon understanding the shock-web complex, for it can give rise to distributions that confound interpretation in terms of the standard model for radiative aging of radio galaxies. 3. The magnetic field outside of the jet itself becomes very intermittent and filamentary in these simulations, yet adiabatic expansion causes most of the cocoon volume to be occupied by field strengths considerably diminished below the nominal jet value. Thus population aging rates vary considerably from point to point.Comment: 44 pages, 6 figures; to be published in the Astrophysical Journal (August 2001); higher-quality figures can be found at http://www.msi.umn.edu/Projects/twj/radjet/radjet.htm

Implications of an r-mode in XTE J1751-305: Mass, radius and spin evolution

Recently Strohmayer and Mahmoodifar presented evidence for a coherent oscillation in the X-ray light curve of the accreting millisecond pulsar XTE J1751-305, using data taken by RXTE during the 2002 outburst of this source. They noted that a possible explanation includes the excitation of a non-radial oscillation mode of the neutron star, either in the form of a g-mode or an r-mode. The r-mode interpretation has connections with proposed spin-evolution scenarios for systems such as XTE J1751-305. Here we examine in detail this interesting possible interpretation. Using the ratio of the observed oscillation frequency to the star's spin frequency, we derive an approximate neutron star mass-radius relation which yields reasonable values for the mass over the range of expected stellar radius (as constrained by observations of radius-expansion burst sources). However, we argue that the large mode amplitude suggested by the Strohmayer and Mahmoodifar analysis would inevitably lead to a large spin-down of the star, inconsistent with its observed spin evolution, regardless of whether the r-mode itself is in a stable or unstable regime. We therefore conclude that the r-mode interpretation of the observed oscillation is not consistent with our current understanding of neutron star dynamics and must be considered unlikely. Finally we note that, subject to the availability of a sufficiently accurate timing model, a direct gravitational-wave search may be able to confirm or reject an r-mode interpretation unambiguously, should such an event, with a similar inferred mode amplitude, recur during the Advanced detector era.Comment: 8 pages, 3 figures; submitted to MNRA

Study of foldable elastic tubes for large space structure applications, phase 1

Structural members that might be suitable for strain energy deployable structures, are discussed with emphasis on a thin-walled cylindrical tube with a cross-section that is called 'bi-convex'. The design of bi-convex tube test specimens and their fabrication are described as well as the design and construction of a special purpose testing machine to determine the deployment characteristics. The results of the first series of tests were quite mixed, but clearly revealed that since most of the specimens failed to deploy completely, due to a buckling problem, this type of tube requires some modification in order to be viable

On the Nature of X-ray Surface Brightness Fluctuations in M87

X-ray images of galaxy clusters and gas-rich elliptical galaxies show a wealth of small-scale features which reflect fluctuations in density and/or temperature of the intra-cluster medium. In this paper we study these fluctuations in M87/Virgo, to establish whether sound waves/shocks, bubbles or uplifted cold gas dominate the structure. We exploit the strong dependence of the emissivity on density and temperature in different energy bands to distinguish between these processes. Using simulations we demonstrate that our analysis recovers the leading type of fluctuation even in the presence of projection effects and temperature gradients. We confirm the isobaric nature of cool filaments of gas entrained by buoyantly rising bubbles, extending to 7' to the east and south-west, and the adiabatic nature of the weak shocks at 40" and 3' from the center. For features of 5--10 kpc, we show that the central 4'x 4' region is dominated by cool structures in pressure equilibrium with the ambient hotter gas while up to 30 percent of the variance in this region can be ascribed to adiabatic fluctuations. The remaining part of the central 14'x14' region, excluding the arms and shocks described above, is dominated by apparently isothermal fluctuations (bubbles) with a possible admixture (at the level of about 30 percent) of adiabatic (sound waves) and by isobaric structures. Larger features, of about 30 kpc, show a stronger contribution from isobaric fluctuations. The results broadly agree with an AGN feedback model mediated by bubbles of relativistic plasma.Comment: 16 pages, submitted to Ap

Propagation of Light Elements in the Galaxy

The origin and evolution of isotopes of the lightest elements H2, He3, Li, Be, B in the universe is a key problem in such fields as astrophysics of CR, Galactic evolution, non-thermal nucleosynthesis, and cosmological studies. One of the major sources of these species is spallation by CR nuclei in the interstellar medium. On the other hand, it is the B/C ratio in CR and Be10 abundance which are used to fix the propagation parameters and thus the spallation rate. We study the production and Galactic propagation of isotopes of elements Z<6 using the numerical propagation code GALPROP and updated production cross sections.Comment: 4 pages, 6 ps-figures, tsukuba.sty, to appear in the Proc. 28th International Cosmic Ray Conference (Tsukuba, Japan 2003). More details can be found at http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm

Refinement and preliminary evaluation of two tablet-based tests of real-world visual function

PURPOSE: To describe, refine, evaluate, and provide normative control data for two freely available tablet-based tests of real-world visual function, using a cohort of young, normally-sighted adults. METHODS: Fifty young (18-40 years), normally-sighted adults completed tablet-based assessments of (1) face discrimination and (2) visual search. Each test was performed twice, to assess test-retest repeatability. Post-hoc analyses were performed to determine the number of trials required to obtain stable estimates of performance. Distributions were fitted to the normative data to determine the 99% population-boundary for normally sighted observers. Participants were also asked to rate their comprehension of each test. RESULTS: Both tests provided stable estimates in around 20 trials (~1-4 min), with only a further reduction of 14%-17% in the 95% Coefficient of Repeatability (CoR95 ) when an additional 40 trials were included. When using only ~20 trials: median durations for the first run of each test were 191 s (Faces) and 51 s (Search); test-retest CoR95 were 0.27 d (Faces) and 0.84 s (Search); and normative 99% population-limits were 3.50 d (Faces) and 3.1 s (Search). No participants exhibited any difficulties completing either test (100% completion rate), and ratings of task-understanding were high (Faces: 9.6 out of 10; Search: 9.7 out of 10). CONCLUSIONS: This preliminary assessment indicated that both tablet-based tests are able to provide simple, quick, and easy-to-administer measures of real-world visual function in normally-sighted young adults. Further work is required to assess their accuracy and utility in older people and individuals with visual impairment. Potential applications are discussed, including their use in clinic waiting rooms, and as an objective complement to Patient Reported Outcome Measures (PROMs)

Film thickness measurements on five fluid formulations by the mercury squeeze film capacitance technique

The thinning characteristics of five fluids were studied by measuring film thickness as a function of time. The mercury squeeze film capacitance technique was used. All tests were performed at room temperature. The synthetic hydrocarbon plus a nematic liquid crystal, N-(p-methoxybenzylidene)-p-butylaniline, thinned according to a Newtonian model and retained its bulk viscosity. The synthetic hydrocarbon plus a phosphonate antiwear additive and the synthetic hydrocarbon plus n-hexadecanol produced residual thick films. The synthetic hydrocarbon base fluids and the synthetic hydrocarbon plus a paraffinic resin displayed viscosity increases during thinning, but no residual films were formed